The in vivo rates of protein synthesis and degradation, and the export of plasma proteins are being determined by a variety of procedures in the mouse liver. Results obtained so far indicate that the net protein gain during liver regeneration can be best explained by a decreased rate of protein degradation. These observations are being extended to other experimental situations. Attempts are also being made in the following directions: 1) to ascertain the validity of the rate of disappearance of (C14-guanido) labelled arginine from liver protein as an accurate measure of the rate of protein degradation. 2) To determine the participation of liver liposome in the higher rates of degradation of proteins in the normal rather than in the regenerating liver.